A spiral welded conical structure that could be made on the construction site could be far more efficient than current designs of support structures such as wind turbine towers. It could result in lighter weight for the same or greater height, compared to traditional towers that are transported to site and bolted together. Existing spiral welding technology is incapable of producing the required conical structure without significant deformation of the metal stock, a process that is impractical to achieve on site.
U.S. Pat. No. 2,038,576 describes forming a cone from a piece of paper cut into an arc segment; however, the method does not allow for multiple wraps as would be required to form a large diameter cone from a single narrow strip of material; nor does the method have any logical extension to evolve into such a required shape as the radial edges of the paper force an axial seam and thus are inherently limiting on the length of the cone that can be formed to be less than the radial length of the edges.
U.S. Pat. No. 2,008,423 describes making a hollow tapered shaft, such as for a golf club, from a uniform or tapered width strip of material. The inventor properly notes that as the strip is wound to form the tapered cone, the helix angle changes which causes the edges to spread apart, and that this is prevented by winding the strip onto a mandrel while under tension which allows the strip to be deformed so the pitch can change without causing buckling of the strip. While suitable for forming a shaft such as golf club, it would not be possible to provide a mandrel for a very large structure, such as a wind turbine tower.
U.S. Pat. No. 3,997,097 describes forming a tapered tube by first feeding a strip between forming rollers that taper the material across its width as a slight curvature is imparted to the strip to enable the helix angle to change as the tapered tube is formed. This patent describes “known prior art” but not U.S. Pat. No. 2,008,423, which describes the critical function of tensioning a constant width strip while forming it over a mandrel although the deformation described is similar. While a mandrel is not required for this case, the order of size of the machine required to form the strip in this manner would make it difficult to realize for very large cones such as that required for large structures such as a windmill tower. Feeding a previously manufactured bent strip to the machine is mentioned, but no algorithm for what the shape needs to be is mentioned.
U.S. Pat. No. 4,082,211 (by the same inventor as U.S. Pat. No. 3,997,097 and using similar figures) describes a process for producing frusto-conical tubes of gradually varying diameter by winding strip stock into a chain of frusto-conical helical turns and joining the edges as they are wound by automatically continuously sensing the variation in tube diameter as it is being produced, automatically governing said variation responsive to rotation of a pair of stock-edge squeezing rollers, and automatically feeding back sensed information of said variation in tube diameter to effect the desired adjustment in spacing between said rollers, and for automatically controlling the rate at which said variation takes place. This will create the in-plane curvature required, but it is not applicable to the large 2 m wide strips needed for large structures, as the rolling forces in the cold state would be titanic and it would not be practical to heat the strip. Furthermore, the described feedback system is only capable of finding the required curvature of the strip during the cone forming process.
U.S. Pat. No. 6,339,945 describes a spiral tube forming system for forming a strip into a spiral tube; a strip in-feed system adapted for feeding a strip to the pipe forming system; and computer-controlled means for continuously varying the angular orientation of the tube forming system relative to the strip in-feed system to selectively vary the diameter of the forming tube. It uses a metal strip of constant width and continuous change in feeding angle to vary diameter. There is no mention of the required geometry to form a cone. There is no mention of cutting the metal strip. The apparatus disclosed does not have the ability to bend the strip in-plane, and even it was to do so, the strip would buckle. To form the strip in-plane curvature, the strip must be very hot, and hence is best form at the mill when it is made. If the strip were provided from the mill with the desired curvature, feeding angle control is necessary as described in U.S. Pat. Nos. 6,732,906, 3,997,097, and 1,914,976.
U.S. Pat. No. 6,732,906 describes a method for meeting the varying helix angle requirement by imparting out-of-plane waviness to one edge of an otherwise constant width strip to impart an overall slight in-plane curvature needed to form the varying helix angle as a tapered cone is formed. However, this could decrease the buckling strength of the tower. Although it uses a metal strip of constant width which is bent in plane using corrugating rolls there is no mention of the required geometry to form a cone, which as shown by the present invention, is a complex non-obvious shape. There is no mention of cutting the metal strip.
For large wind turbine towers, for example, it will be desirable to form tapered (conical) towers on-site so the base can be very large in diameter. Prior art to this effect includes U.S. Pat. No. 3,030,488 where a metal strip spool rests on its own weight on its driving assembly. The method describes sensing the strip edge position before the weld and creating a feedback loop including means to vary the feed-in angle. A stated goal of this method is to decrease the number of elements and thus provide a “transportable machine by which tubes can be welded at the point of use” [1-29].
The need for taller towers with larger diameter bases is discussed below in the context of FIGS. 9 and 10. One way that industry has currently been trying to meet this need is through segmented designs. Currently, cylindrical segments are used, but to get to large diameters, the circular segments can be further segmented into arcs. As an example, see U.S. patent application “Method of constructing large towers for wind turbines (2003, application Ser. No. 10/549,807): “In order to transport large size windmill towers, the invention suggests a steel tower (1) for a windmill, comprising a number of cylindrical or tapered tower sections (2), at least the wider sections (2) of which being subdivided into two or more elongated shell segments (3), which combine into a complete tower section (2) by means of vertical flanges (6) tightened together, e.g., by bolts (10), said shells being also provided with upper and lower horizontal flanges (4), respectively, to allow interconnection of tower sections (2) one on top of the other.” [Abstract]. However, this results in great complexity and an enormous number of bolts to be installed and then periodically checked.